Thermal inkjet printheads include an array of ink firing chambers having openings from which ink is projected onto a sheet of paper or other medium. Each ink firing chamber is aligned with a thermal actuator, i.e., a resistive heater. Current flow through the actuator causes a portion of the ink within the firing chamber to vaporize and eject an ink drop through the opening. The openings are arranged in linear arrays along a surface of the printhead.
With reference to FIG. 1, a prior art thermal inkjet printhead is schematically shown as including a silicon substrate 10 and a polymer barrier layer 12. Formed on the silicon substrate is a resistor layer 14 and a metallization layer 16. The resistor layer is patterned to define dimensions and locations of ink firing actuators 18. While not shown in FIG. 1, the metallization layer extends beyond the actuator and provides an electrical path for control signals to the actuator. A passivation layer 20 is disposed over the metallization layer, and the polymer barrier layer 12 is attached to the passivation layer. The polymer barrier layer is patterned to include an ink firing chamber that exposes the thermal actuator 18. The barrier layer 12 includes an open side 22 that is in fluid communication with an ink supply channel.
Referring now to FIGS. 1 and 2, atop the barrier layer 12 is an orifice substrate 24 having an opening 26. In practice, the barrier layer 12 is often formed in conjunction with the orifice substrate 24. The opening 26 defines the geometry for firing ink from the inkjet mechanism in response to activation of the thermal actuator 18. The actuator is individually addressed by means of a switching transistor 28 connected to the actuator by a conductive trace 30.
In operation, current flow through the thermal actuator 18 is initiated by the electronic circuitry 28. As the actuator heats, a vapor bubble is formed in the firing chamber and a pressure field is generated. As a result, ink is projected from the firing chamber toward a medium, such as a sheet of paper. The firing chamber is replenished with ink by flow from a supply channel 32 of the silicon substrate 10. The ink enters the firing chamber through the open side 22 of the barrier layer 12.
As explained in U.S. Pat. No. 5,450,109 to Hock, which is assigned to the assignee of the present invention, the conventional method of fabricating inkjet printheads is to utilize photolithographic techniques to form the thermal actuators 18 on the silicon substrate 10. Separately, the ink firing chambers are photolithographically defined within the polymer barrier layer 12 that is formed on the orifice substrate 24. The orifice substrate may be formed of a gold-plated nickel material. The orifice substrate and barrier layer are then attached to the actuator substrate 10 with the firing chambers in precise alignment with the actuators.
Utilizing conventional fabrication techniques, the inkjet printhead includes three structures, i.e., the silicon substrate with the thermal actuators, the barrier layer in which the ink supply channels and firing chambers are formed, and the orifice plate having the openings for the projection of ink. Often, the manufacturing process includes adhering two substrates together to provide the final product. Adhering the substrates in order to provide the desired architecture raises concerns with respect to reliability, cost, manufacturability and print quality. Improved print quality requires smaller ink drop volumes and, therefore, smaller ink firing chambers and openings. As ink firing chambers and thermal actuators are reduced in size, it becomes increasingly difficult to properly align the array of ink firing chambers on one substrate with the array of thermal actuators on another substrate. Limits imposed by the ability to repeatedly and reliably align the two substrates are factors in dictating the throughput, cost and print quality available using inkjet technology. Another limitation of the bonded structure stems from the fact that adhesives tend to fail due to long-term exposure to aggressive inks and thermal cycling. Repeated heating and cooling, as well as contact with chemically aggressive inks, often cause degradation of the polymer barrier layer and loss of adhesive properties. Partial or total delamination of the orifice substrate from the actuator substrate may result.
U.S. Pat. No. 5,412,412 to Drake et al. describes the procedure for bonding the substrates as being paramount to maintaining the efficiency, consistency and reliability of an inkjet printhead. The alignment and bonding process described in Drake et al. includes introducing elements into the fabrication sequence to compensate for any topographical formations that are developed in a thick film insulating layer during fabrication. The insulating layer is formed to intentionally include a non-functional heater pit and a non-functional bypass recess. The non-functional features are on opposite sides of arrays of functional heater pits and bypass recesses. In like manner, a silicon substrate is formed to include non-functional grooves that are positioned to straddle topographical formations formed proximate to the non-functional heater pits and bypass recesses formed in the insulating layer. Therefore, the topographical formations do not cause the silicon substrate to stand off from the thick film insulating layer.
Another patent that addresses the process of connecting two substrates in forming an inkjet printhead is U.S. Pat. No. 5,388,326 to Beeson et al., which is assigned to the assignee of the present invention. The first substrate includes inkjet nozzles and an array of conductive traces that are formed in a preselected pattern. The second substrate is a "die layout" having a barrier material, an array of resistors formed in wells within the barrier material, and an array of channels formed in the barrier material. The positions of the resistors and the channels of the die layout match the positions of the inkjet nozzles and the conductive traces, respectively. By interlocking the conductive traces with the channels, the resistors are aligned with the inkjet nozzles. The first substrate and the barrier material are then laminated so as to bond the two together.
While the prior art techniques for bonding substrates of an inkjet printhead provide acceptable results, further improvements are desired in order to accommodate advancements with respect to print quality, printhead reliability, manufacturing throughput, and cost reduction. Moreover, a major source of printhead failures continues to be delamination of the orifice substrate from the actuator substrate. As previously noted, the substrate-to-substrate bonds tend to fail due to the long-term exposure to thermal cycling. U.S. Pat. No. 5,016,024 to Lam et al. provided a degree of improvement by forming heaters adjacent to the orifices on an orifice plate. An ink reservoir wall is connected in parallel with the orifice plate. An ink heating zone for a particular orifice is provided by a gap between the ink reservoir wall and the orifice plate. Electrical current through a heater rapidly heats the volume of ink in the adjacent ink heating zone, forming a bubble for projecting ink through the orifice. While the Lam et al. printhead reduces substrate-to-substrate alignment requirements, substrate delamination remains a concern, since the ink heating zone still includes the zone between the orifice plate and the bonded substrate. Another concern relates to the spatial relationship between a heater and an associated orifice. The thermal transfer is at a 90 degree angle to the direction of ink projection. This relationship may adversely affect either or both of the efficiency and the reliability of a firing operation. Furthermore, if the electronic circuitry for controlling ink firing is fabricated onto the ink reservoir wall, there must be hundreds of electrical connections that extend from the ink reservoir wall to the large number of heaters on the orifice plate.
What is needed is an inkjet printhead and fabrication method in which the alignment of an array of ink firing chambers with an array of actuators, such as thermal actuators, is precisely and repeatedly achieved. What is further needed is an inkjet printhead that is less susceptible to long-term failures than printheads that are fabricated by conventional approaches of adhering printhead components together with polymers.